Power converter

ABSTRACT

A switch-type power converter includes a double FET switch operating in a variable duty cycle mode under the control of a Unitrode 3846 integrated circuit controller. Indications of excess input voltage and reverse battery connections are provided by circuits including an element which permanently changes state. A cooling fan mounted on a finned heat sink is operated in a variable speed mode.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 11/221,962filed under attorney docket no. PDY-112-A on Sep. 8, 2005, currentlypending. The content of the U.S. patent Ser. No. 11/221,962 isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to power converters, and more particularly toswitch-type power converters designed for use in recreational vehiclesas a regulated power supply for DC load devices and as a batterycharger.

BACKGROUND OF THE INVENTION

As used herein, the terms “recreational vehicle” and “RV” should beconstrued to embrace motor homes, trailers, campers, van conversions,fifth wheels, boats, and similar products. The common characteristic ofthese recreational vehicles is an electrical system incorporating one ormore batteries to provide power for DC load devices, such as lights,refrigerators and motors. The more sophisticated recreational vehiclesmay also have alternating current systems and AC load devices such asstoves, televisions, microwaves and heating and ventilating systems. TheAC load devices are typically powered, from, a 115 volt AC line voltagesource brought to the recreational vehicle through a power cord andplug. Some recreational vehicles also carry generators powered by gas ordiesel engines and capable of producing as much as 20 or more kilowattsof AC power.

It has become common to install power converters in recreationalvehicles. A typical power converter converts 115 vac to 13.6 vdc andcharges the RV battery or batteries as necessary. It has become more andmore common to use “switch type” power converters rather than linearconverters. There are numerous reasons for this including a substantialweight savings. Switch type power converters, often simply called“switchers” or “switching” power converters, typically use one or twopower switching semi-conductor devices such as field effect transistors(“FET's”) and a controller such as the Unitrode, UC 3846 for operatingthe semi-conductor devices in a variable duty cycle mode. Such devicesfurther typically include a step-down transformer and a smoothingcircuit between the transformer and the regulated output voltageterminal.

A designer of such converters faces numerous issues including heatdissipation, noise generation, tolerance to unstable or excessive supplyvoltages and protection of the expensive circuit components foundtherein. The manufacturer of such devices faces these and other issuesincluding warranty claims based on alleged defects when, in fact, fieldfailures are often caused by improper use such as (1) accidentallyconnecting the converter input to an excessive voltage source such as a220 vac line or an improperly regulated or runaway generator; and (2)accidentally connecting the RV battery in reverse polarity

Power converters which deal with some of these issues are described inU.S. Pat. Nos. 5,600,550 and 5,687,066 issued to James Cook in Februaryand November, respectively, of 1997 and assigned to ProgressiveDynamics, Inc. of Marshall, Mich. The power converter described in the'550 patent is of the switch type in which the switch includes two FET'soperating in a push/pull fashion under the control of an integratedcircuit controller such as the Unitrode UC 3846. The converter furthercomprises a fan powered by the converter output and a pair ofthermistors mounted on a large heat sink along with the FET's. One ofthe thermistors is used in combination with a set-point device to turnthe fan on and off and the other is used to shut the controller down inthe event temperature reaches an extreme or intolerable level.

U.S. Pat. No. 5,687,066 describes a converter identical to that of the'550 patent but adds overvoltage protection. This feature is provided bya Zener diode to sense an overvoltage condition in the dc output of adiode rectifier bridge used to convert an ac line voltage to dc. If therectified supply voltage exceeds a predetermined limit, the Zener diodeconducts and quickly sends a signal to a shut down pin of the Unitrodecontroller to prevent the controller from turning the FET's on. Thisprotects the FET's from damage until the overvoltage condition subsides.

SUMMARY OF THE INVENTION

The subject invention has for its foundation a switch-type powerconverter/battery charger including a switch consisting of one or moreFET's operating in a variable duty cycle mode. An integrated circuitcontroller such as the Unitrode UC 3846 is used with appropriatefeedback and a rectifier and LC filter in the output stage to operatethe switch to produce a regulated dc output.

The subject converter in a typical commercial embodiment includes arectifier bridge so that the unit may be connected to a standard 60cycle normal 115 volt ac line. This is typical of the line voltage madeavailable by electric utility companies and/or commercial generators.The feedback system is used to cause the overall converter to operate ina current demand mode wherein the duty cycle of the switch is adjustedto maintain the desired output voltage.

In the preferred embodiment described herein, the converter furthercomprises a transformer for stepping voltages within the convertercircuit down to a level suitable for use in connection with dc loaddevices and the charging of conventional storage batteries. Most of thereference voltages in the converter are taken from the primary side ofthe transformer. In addition, the fan supply and fan control are on theprimary side of the transformer. By supplying the fan from the primaryside, an undesirable drop in fan speed under heavy load conditions isavoided.

According to a first, more specific aspect of the present invention, acircuit is provided at or near the dc input of the converter i.e., at ornear the output of the ac-to-dc rectifier circuit, for providing apermanent indication of an abnormal over-voltage condition sufficient tocause circuit damage and likely to be the result of operator error. Ingeneral, the permanent over-voltage indicator comprises a circuitconnected between the output of the ac-to-dc rectifier and ground andincludes a device, such as a Zener diode, for establishing a very highbreakdown voltage, and a device, such as a fuse, which permanentlychanges state in response to an over-current condition. The fuse andZener diode are preferably chosen in the commercial embodiment tocorrespond to the conditions which might exist if the converter wereaccidentally connected to a 220 volt ac supply or to an unregulated orrunaway generator. The permanent change of state in itself has no effecton converter operation, since it is not a shut down mechanism similar tothat of the over-voltage protection feature. But it does provide themanufacturer or warrantor of the system with evidence that any damageoccurring to the converter and/or its various circuit components was theresult of an extreme over-voltage condition rather than systemmalfunction or component defects.

The permanent input over-voltage indicator is preferably used incombination with an over-voltage shutdown circuit also connected to theoutput of the ac-to-dc rectifier. The location and overall purpose ofthe over-voltage shutdown circuit is generally as described in the '066patent where it is referred to as an overvoltage “protection” circuit,but preferably uses an operational amplifier to establish the shutdownset point voltage in a way which is more precise than that availablefrom the use of a Zener diode as described in the '066 patent. Theoutput of the over-voltage shutdown circuit is connected to a shut downpin in the variable duty cycle controller so as to prevent the switchtransistors from turning on (and off again) while the over-voltagecondition persists. This protects the expensive FET's and othercomponents in the switch from damage. The set point of the over-voltageshutdown circuit in the illustrated embodiment is lower than thatassociated with the permanent, over-voltage indicator device describedabove and the two circuits work in a cooperative fashion; i.e., theover-voltage shutdown circuit effects a shut down function at a firstover voltage level whereas the permanent over-voltage indicator circuitchanges state at a substantially higher over-voltage level likelyresulting from, for example, owner/user error or generator runaway.However, the trip point of the overvoltage indicator could be set belowor equal to the overvoltage shutdown circuit if the circuit designerwishes to do so.

Another aspect of the present invention in the foundation environmentdescribed above is a permanent reverse battery connection indicatorcircuit. This circuit detects a so-called “reverse” battery conditionwhich results from the erroneous reverse polarity connection of thestorage battery to the recreational vehicle electrical system after aperiod of disconnection for storage or service. Like the over-voltageindicator, the permanent reverse battery connection circuit includes acomponent which undergoes a permanent change of state when the batteryis accidentally connected with the positive and negative terminals inreverse positions. Again, the permanent indicator does nothing to shutdown or disable system operation, but simply provides an unequivocalindicator of owner/user error in the event a warranty claim is latermade.

The converter of the present invention, like the converter described inthe '066 patent, uses a metal heat sink as part of the converterpackaging structure and mounts certain components on or in contact withthe heat sink. A thermistor sensor, preferably mounted on or in contactwith the heat sink, is used to monitor converter temperature and providean output signal which; also unlike the '066 patent converter, issimultaneously supplied to two control circuits. The first controlcircuit operates the fan in a variable speed mode. These modes ofoperation are believed to not only extend fan life, but also reduce anannoying quality of fan noise. The thermistor sensor also furnishes atemperature-related signal to a second circuit including a comparator or“op-amp” to shut down the variable duty cycle controller in the event ofa high temperature condition which may exceed the capacity of the fan.

Other aspects of the invention in the area of thermal control include aspecial mounting arrangement between the fan motor and the extrusionwhich provides the heat sink; i.e., a recess is machined into an end ofthe heat sink extrusion to provide an air gap between the extrusion andthe fan motor so that the fan motor does not directly pick up heat fromthe extrusion. In addition, heavy wire leads are used in overlyingrelationship to the copper plating of a circuit board used to mount theelements of the circuit of FIG. 2. The wire leads are soldered to theboard in high current connector areas. Numerous advantages flow fromthese packaging modifications as will be hereinafter explained ingreater detail.

Still further aspects and advantages of the invention are describedherein and will be best understood from a reading of the followingspecification which describes and illustrative embodiment in the form ofan 80 amp power converter for use in recreational vehicles of the typeusing conventional storage batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a switched power converter circuitaccording to the present invention;

FIG. 2 is a schematic circuit diagram of an illustrative switched powerconverter circuit embodying the inventive features described above;

FIG. 3 is a graph of temperature versus fan speed illustrating theoperating curve of the fan according to the present invention;

FIG. 4 is a graph of fan voltage versus fan speed for a typical fan;

FIG. 5 shows various waveforms within the circuit of FIG. 17;

FIG. 6 is a graph of output converter current versus temperature for avariety of fan speeds;

FIG. 7 is a partial schematic diagram illustrating atemperature-responsive input circuit according to the present invention;

FIG. 8 is a graph of V_(tempvar) of FIG. 7 versus fan voltage showingthe desired characteristic;

FIG. 9 shows partial schematics of a fan connected to an operationalamplifier;

FIG. 10 shows a partial schematic of a fan connected to an opencollector operational amplifier and a graph showing the resulting fanvoltage curve with temperature changes;

FIG. 11 shows the partial schematic of FIG. 10 with the addition of again amplifier and a graph showing the resulting fan voltage curve withtemperature changes;

FIG. 12 shows the partial schematic of FIG. 11 with the addition ofcircuit to shift the zero point of the fan voltage curve and a graphshowing the resulting fan voltage curve with temperature changes;

FIG. 13 shows the equivalent circuit to the circuit to shift the zeropoint of FIG. 12;

FIG. 14 shows the equivalent circuit to the open collector operationalamplifier of FIG. 12;

FIG. 15 is a schematic of a first embodiment of the control circuitaccording to the present invention;

FIG. 16 is a schematic of a second, alternative, embodiment of thecontrol circuit according to the present invention;

FIG. 17 illustrates the two output current paths generated by thesecondary-output side of transformer T1;

FIG. 18 is a perspective view of a fully packaged power converterembodying the features described herein;

FIG. 19 is a cross-section of an illustrative heat sink showing a springclip to hold a diode in the switch circuit against the heat sink;

FIG. 20 is an end elevational view of the power converter package ofFIG. 18;

FIG. 21 is an opposite end elevational view of the power converterpackage of FIG. 18;

FIG. 22 is a top plan view of the switched power converter package ofFIG. 18;

FIG. 23 is a perspective view of a RV partially broken away to show theswitched power converter according to the invention positioned therein;

FIG. 24 is a perspective view of the heat sink of FIG. 19 showing arecess or relief in the fan mounting surface.

FIG. 25 is a photograph of one side of the circuit board used to supportthe components in the circuit of FIG. 2 showing heavy wires connectedfrom the center top of the transformer through the circuit board; and

FIG. 26 is a photograph of the reverse side of the circuit board, withthe image reversed to coincide with the orientation of the FIG. 25photograph, showing the heavy wires from the transformer coming throughthe circuit board and soldered over the conductive traces leading to thenegative output terminal. This photograph also shows additional heavywires running from the fuses to the positive output terminal and alsosoldered to and in overlying relation to circuit board traces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a circuit for a switched power converterembodying the features of the present invention. The block diagramincludes an AC-to-DC rectifier circuit 10, a switch circuit 12, atransformer circuit 14, a feedback circuit 16, a controller 18, anover-voltage shutdown circuit 20, a permanent over-voltage indicator 22,a permanent reverse battery indicator 24, a thermistor circuit 26, avariable speed fan 32, fan control circuit 30, an, over-temperatureshutdown circuit 28, a current sensing feedback circuit 34, a foldbackcircuit 42, and an output rectifier and LC filter circuit 44 includingthe inductor L2 referred to hereinafter.

The AC-to-DC rectifier circuit 10 converts a 115v AC line voltage intoan unregulated and time-varying dc signal with an average in the 170volt range. It should be noted that the converter 46 can be plugged intoa 170vdc source, if available. In this case the rectifier 10 performs norectification functions. The unregulated DC signal then enters theswitching circuit 12 where the on/off states and duty cycle of theswitching circuit 12 is determined by the controller 18 and feedbackcircuits 34 and 16. The switching circuit 12 includes two field effecttransistors (FET's). The output of the switching circuit 12 is aregulated waveform containing unidirectional pulses.

Current sensing feedback circuit 34 is connected to the output of theswitching circuit 12 for the purpose of measuring the output current.The output of the current sensing circuit 34 is connected to controller18. Controller 18 adjusts the duty cycle of the FET's in the switchingcircuit 12 according to the current measured by the current sensingcircuit 34 and the voltage measured by circuit 16. Accordingly, dutycycle is controlled by two factors: voltage feedback via circuit 16 andcurrent feedback via circuit 34.

Over-voltage shut-down circuit 20 is connected between the output of theAC-to-DC rectifier circuit 10 and a shut-down pin of the controller 18for the purpose of shutting off the switching circuit 12 in the eventthe rectified input voltage at 40 exceeds a pre-determined thresholdvoltage such as 195 vdc. The permanent over-voltage indicator 22 isconnected to the output of the AC-to-DC rectifier circuit 10 for thepurpose of triggering a permanent indicator in the event the voltage at40 exceeds a second, higher threshold voltage, such as 220 vdc. As notedabove, the second threshold voltage will typically be higher than thefirst, but could be lower or equal to the first threshold voltage. Theover-voltage shut-down circuit 20 will protect the costly transistorcomponents of the switching circuit 12 from being destroyed by theexcessive input voltage conditions. The permanent over-voltage indicator22 will provide evidence to the manufacturer that an undesirably high ACinput voltage had been connected to the converter, e.g., a 220 VAC linevoltage. The threshold voltage triggering the over-voltage shut-downcircuit 20 is typically lower than the threshold voltage triggering thepermanent over-voltage indicator 22, but can be higher or equal to theovervoltage indicator circuit trigger voltage.

The regulated signal passes from the switching circuit 12 to thetransformer circuit 14. The transformer circuit 14 steps down theaverage of the unidirectional pulses to the level necessary forrecreational vehicle use; e.g., ultimately to about 13.6 volts. Thestepped down waveform is rectified and smoothed by circuit 44 beforeapplication to load devices. Feedback circuit 16 measures the voltageacross the load. The output of the feedback circuit 16 is connected tocontroller 18. Controller 18 then controls the on/off state and dutycycle of the switching circuit 12 based in part on the input receivedfrom the feedback circuit 16.

A permanent reverse battery indicator 24 is also connected across the DCload for the purpose of providing a physical record that the operatorconnected a battery in reverse polarity. Such reverse batteryconnections may cause damage to the switched power converter, and themanufacturer may have an interest in knowing whether the damage wascaused by the reverse connection of the RV battery as opposed to amanufacturing defect.

Thermistor circuit 26 senses the temperature of a heat sink 52 in thehousing 70, and provides a variable resistance based on temperature.Over-temperature shutdown circuit 28 receives a signal from thethermistor circuit 26 and, if a set-point is exceeded, sends a shutdownsignal to the controller 18. Controller 18 then terminates the operationof switching circuit 12. The over-temperature shutdown circuit 28 willnot permit the operation of the switching circuit 12 until thetemperature sensed by the thermistor has fallen below the undesirabletemperature limit. Hysteresis in the circuit makes the temperature atwhich operation is resumed lower than the shutdown temperature.

Fan control circuit 30 receives a signal from thermistor circuit 26. Thefan control circuit 30 produces a variable output based on the inputfrom the thermistor 26. A variable speed fan 32 is connected to thevariable output signal of the fan control circuit 30, such that the fan32 will vary in speed based on the input signal. Accordingly, the speedof the fan 32 increases in response to increases in sensed temperatures.A low fan speed minimizes the annoying effects of fan noise at low tomoderate power levels. The power supply for the fan 32 comes from theprimary side of transformer circuit 14. This feature eliminates thetendency of the fan supply voltage to droop, with a corresponding fanspeed reduction, under heavy load conditions.

Having briefly described the overall block diagram of the switched powerconverter circuit, the schematic circuit of an illustrative, mechanicalembodiment will be described in detail with reference to FIGS. 2, 5 and17. The preferred values of all described electrical components arelisted at the end of the detailed description.

Input Circuit

Input circuit 36 is connected to a conventional AC power supply througha cable having a conventional 3-prong connector. The 3-prong connectorincludes a ground conductor, a positive conductor, and a neutralconductor. The cable runs into the housing through an aperture 100. TheAC positive terminal is connected to AC positive input W1. The AC groundterminal is connected to AC ground W2 identified by a “chassis ground”symbol. The AC neutral terminal is connected to AC neutral input W3. ACpositive input W1 is connected to thermistor RT2. Thermistor RT2 is usedas an inrush current protector for the purpose of protecting fullydischarged capacitors from receiving a surge of current. Thermistor RT2initially (i.e. when cold) provides a high resistance but rapidlychanges to a substantially lower resistance as the temperatureincreases, allowing an unrestricted AC signal to pass into the noisesuppression circuit 38. It should be noted that there is a primaryground, secondary ground and a “chassis” ground and that differentsymbols are used for these in FIG. 2.

Noise Suppression Circuit

The noise suppression circuit 38 includes capacitors C15, C16, C26, C1,C2, C3, C30, C29, and C31, inductor beads L5, L6, L7, and L8, jumpersJ6, J7, J8, J9, J10, and J11, and common mode choke (CMC) transformersT3, and T2. These electrical components provide electromagneticinterference noise suppression, and filtering to prevent noise fromwithin the converter from traveling back into the ac supply line. Noisetransfer suppression is also provided by capacitors C15, C16, and C26.One plate of C15 is connected to thermistor RT2, and one plate of C16 isconnected to AC neutral input line W3. The other plates of capacitorC15, and C16 are connected to chassis ground, i.e., the ground of inputW2. Capacitor C26 is connected in parallel to C15 and C16, where oneplate of capacitor C26 is connected to thermistor RT2, and the otherplate of capacitor C26 is connected to AC neutral input line W3. Bothplates of capacitor C26 are connected through CMC transformer T3.Winding 2-1 of CMC transformer T3 is connected to thermistor RT2, andwinding 3-4 of CMC transformer T3 is connected to AC neutral input lineW3. The output of winding 2-1 is connected to the input side of fuse F1.

Additional noise suppression is provided by capacitors C1, C2, and C3.One plate of capacitor C1 is connected to the output side of fuse F1,and the remaining plate of capacitor C1 is connected to winding 3-4 ofCMC transformer T3. Capacitor C1 is also connected to one plate of eachof capacitors C2 and C3. The remaining plates of capacitors C2 and C3are connected to chassis ground. CMC transformer T2 is connected inparallel to capacitors C2 and C3. Winding 2-1 of CMC transformer T2 isconnected to the ungrounded plate of capacitor C2, and winding 3-4 ofCMC transformer T2 is connected to the ungrounded plate of capacitor C3.Jumpers J6, J7, J8, are connected in parallel to winding 2-1 of CMCtransformer T2, and jumpers J9, J10, and J11 are connected in parallelto winding 4-3 of CMC transformer T2. All jumpers provide the option ofbypassing CMC transformer T2.

Additional noise suppression is provided by capacitors C30, C29, andC31. The windings of CMC transformer T2 are connected in parallel tocapacitor C30. Capacitor C30 is also connected to one plate of each ofcapacitors C29 and C31. The remaining plates of capacitors C29 and C31are connected, to chassis ground. High frequency noise suppression isprovided by inductor beads L5, L6, L7, and L8. L6 and L8 are fitted ontothe bridge connection wires by causing the wires to pass through thecenter opening of each inductor bead core, wrap around the bead core,and then pass again through the bead core. The wires passing through L6and L8 are then connected between T2 and a diode bridge DB1 forming theAC-to-DC rectifier 10. The inductor beads L5 and L7 are similarlymounted on the wires coming out of the diode bridge DB1 (see FIG. 2).

AC-to-DC Rectifier

The AC-to-DC rectifier 10 is composed of a diode bridge DB1, capacitorsC4 a, C4 b, and C4 c. The wires passing through L6 and L8 are connectedto the input of diode bridge DB1. The wires passing through L5 and L7are connected to the output of diode bridge DB1. Capacitors C4 a, C4 b,and C4 c are connected in parallel between wires passing throughinductor beads L5 and L7. The wire passing through L5 is connected tothe positive plate of each capacitor C4 a, C4 b, and C4 c, and the wirepassing through L7 is connected to the negative plate of each capacitorC4 a, C4 b, and C4 c. The negative plates of capacitors C4 a, C4 b, andC4 c are also connected to ground. Under optimal conditions capacitorsC4 a, C4 b, and C4 c are charged by the output of diode bridge DB1 to adesired voltage of 170 volts. Capacitors C4 a, C4 b, and C4 c provide anunregulated DC signal to unregulated DC terminal 40.

Permanent Over-Voltage Indicator

The permanent over-voltage indicator 22 includes fuse Fx1 and Zenerdiode D23 connected in series between the output 40 of rectifier bridgeDB1 and ground. The permanent over-voltage indicator 22 receives thevoltage developed across capacitors C4 a, C4 b, and C4 c, and causes thefuse to change state if the voltage across the capacitors reaches anundesirably high level. The cathode of Zener diode D23 is connected tothe output of fuse Fx1 and the anode of Zener diode D23 is connected toground. It will be noted in FIG. 2 there are three different groundsymbols. One is chassis ground, connected between C2 and C3 forinstance. Another is primary circuit ground, connected to pin 12 of U1for instance. And lastly there is a secondary circuit ground, connectedto the output P1 terminal for instance. Each of these three groundsymbols refer to separate voltage reference points and are isolated fromeach other. It will be further noted that the primary ground symbol issubdivided into an S, S2 and P ground. The explanation is:

S is the signal ground

S2 is the current sensing circuit ground

P is the power ground

In practice, these grounds are separate except at one point in circuitboard layout to avoid parasitic noise cross talk. Nevertheless,physically they are the same since they are connected by copper tracesand wires. The same is the case for the S and P shown with secondaryground symbol.

The purpose of the permanent over-voltage indicator 22 is to provide apermanent indication of receiving an undesirably high input voltagegreater than that which triggers the over voltage shut down circuit 20.If the permanent over-voltage indicator 22 changes state, it will bebecause the converter input receives an excessive voltage, caused, forexample, by a 220vac supply or runaway generator. If enough voltage isapplied to Zener diode D23 it will fail short creating a directconnection between fuse Fx1 and ground. This short failure of D23 causesFx1 to permanently change state, i.e., blow out to create an opencircuit. The preferred voltage limit of the permanent over-voltageindicator 22 is normally 220 volts dc. It will be noted that because theindicator circuit 22 is a shunt, failing the diode and blowing the fuseFx1 does not disable the converter. The term “permanent” is used hereinto mean device which does not reset by itself, i.e., it must be replacedto operate a second time. Since tripping the indicator does not shutdown the converter, the owner has no reason to replace it and typicallywill not be aware of its presence. Therefore, it remains in theconverter until the converter is returned for service or a warrantyclaim. For the majority of converters, this never happens. However, forthe small percentage of converters returned for a warranty claim, theindicator helps the manufacturer evaluate the likelihood that circuitfailures are the result of excessive input voltage other thanmanufacturing or material defect. If a converter is returned for serviceand the indicator fuse Fx1 is failed, it will be replaced along with anyother failed components and may, for example, signal the need to providethe owner with a cautionary message regarding the quality of the supplyvoltage source being used.

Over-Temperature Shut-Down Circuit

The over-temperature shutdown circuit 28 measures the heat sinktemperature in the switched power converter and triggers a shutdown ofthe switching circuit 12 upon receiving an undesirably high temperature.The over-temperature shutdown circuit 28 includes Schottky diode D3,resistors R8, RN1B, R7, and RN1A, operational amplifier U3A, andthermistor RT1. Thermistor RT1 changes in resistance based on sensedtemperature. Preferably thermistor RT1 is anegative-temperature-coefficient device and is mounted on or in contactwith the converter heat sink 52 in the manner shown in FIG. 19; i.e., aspring clip holds the sensor against a surface of the casting whichmakes up the sink 52. Because the FET's in the switch 12 are alsomounted in contact with the sink 52, heavier load conditions cause thetemperature of the sink 52 to rise. If turning the fan 32 on stabilizesthe temperature, no further remedy is needed. It should be noted thatthe thermistor RT1 does not have to be mounted on the heat sink, but canbe mounted to measure, for example, air temperature or the temperatureof some component such as the transformer 14 or the output inductor incircuit 44. The illustrated arrangement is, however, preferred.

Operational amplifier U3A is used as a comparator for the purpose oftriggering shutdown pin (pin 16) of controller 18 in the event that theinternal temperature of the switched power converter exceeds a set-pointtemperature. Once shutdown pin (pin 16) of controller 18 is triggeredthe operation of switching circuit 12 is terminated.

Operational amplifier U3A includes the following connections: pin 1 isthe output, pin 2 is the negative input, pin 3 is the positive input,pin 4 is connected to a 5 volt reference voltage 5REF, and pin 11 isconnected to ground. Pin 2 is connected to a temperature based variablevoltage coming from a voltage divider circuit comprised of resistor RN1Aand thermistor RT1. Pin 3 is connected to a reference voltage through avoltage divider circuit using resistors R8, R7, and RN1B. Pin 1 isconnected to resistor R8, and Schottky diode D3 leading to shutdown pin(pin 16) of controller 18.

The output of operational amplifier U3A will remain at a low (ideallyzero) voltage and will not trigger shutdown pin 16 of controller 18 aslong as pin 3 input does not exceed the pin 2 input. When the internaltemperature is sufficiently high, the voltage on pin 3 will exceed thevoltage on pin 2 and the output of pin 1 will go high and trigger ashutdown.

The over-temperature shutdown circuit 28 will operate as follows under acold temperature condition (i.e. a temperature condition where a thermalshutdown is not required). Resistor RN1A and thermistor RT1 form avoltage divider circuit. Resistor RN1A is connected to a 5 voltreference 5REF and thermistor RT1 is connected to ground. Thus, pin 2receives the voltage between resistor RN1A and thermistor RT1.Accordingly, the voltage applied to pin 2 will vary depending on thetemperature of the heat sink 52.

The value of resistor RN1A is 16.2K ohms, and the value of thermistorRT1 is 100K ohms of 25° C. Thus, when the switched power converter isinitially turned on and the temperature is cold the value of thermistorRT1 will be about 100K ohms. At cold startup the voltage applied to thepin 2 of operational amplifier is roughly 4.3 volts. Further, at a cold(i.e. non thermal shutdown) temperature pin 1 will be near 0 voltsbecause the voltage at pin 2 is higher than the voltage at pin 3. Whenthe voltage at pin 1 is near 0 volts, resistor R8 is parallel withresistor R7.

In the illustrative embodiment, the values of resistors R8, R7 and RN1Bare 499K, 32.4K, and 47.5K ohms respectively. Because resistors R8 andR7 are in parallel, their equivalent resistance at 25° C. is 30.4K ohms.This resistance of 30.4K ohms will be called R_(coldtemp). Accordingly,the voltage at pin 3 will be the measured voltage between resistor RN1Band R_(coldtemp). Using a voltage divider, the voltage applied to pin 3at a cold temperature is 1.925 volts. This voltage will be calledV_(coldtemp). Accordingly, at a cold temperature the voltage at pin 3will be V_(coldtemp) which is 1.925 volts. If the internal temperaturesignificantly increases, the resistance of thermistor RT1 will decreaseand the voltage applied to pin 2 will fall below the voltage applied topin 3, the output of pin 1 will become positive, and the switched powerconverter will experience a thermal shutdown.

The over-temperature shutdown circuit 28 will operate as follows under athermal shutdown condition (i.e. a temperature condition where anover-temperature shutdown is required). A shut down temperature is neverreached if the load on the converter is within normal specificationsbecause the fan 32 will provide sufficient cooling. If the load is veryheavy and/or the operator has covered the converter 46 with blankets orthe like, a shut down temperature may be reached. If this happens, thevoltage applied to pin 1 will be approximately 5 volts. When pin 1reaches 5 volts, resistors RN1B and R8 will be in parallel (as opposedto resistor R7 being in parallel with resistor R8 at a coldtemperature). The equivalent resistance of resistors RN1B and R8 inparallel is 43.37K ohms. This resistance will be called R_(hottemp).Accordingly, the voltage at pin 3 will be the measured voltage betweenresistor R7 and R_(hottemp). Using a voltage divider the voltage appliedto pin 3 at a cold temperature is 2.138 volts. This voltage will becalled V_(hottemp). Accordingly, in order for the switching circuit 12to begin operation the voltage on pin 2 must rise above V_(hottemp)(rather than V_(coldtemp)). This hysteresis caused by resistor R8 isimportant so that the switching circuit 12 will not be enabled until theinternal temperature falls significantly below the temperature at whichthe thermal shutdown was triggered.

Over-Voltage Shutdown Circuit

The over-voltage shutdown circuit 20 measures the voltage of capacitorsC4 a, C4 b, and C4 c, and will shutdown the switching circuit 12 in theevent of an over-voltage condition at point 40. The over-voltage circuit20 includes resistors R38, R39, R40, R7, and RN1B, operational amplifierU3B, and Schottky diode D27. The output of over-voltage shut downcircuit 20 is connected to shutdown pin 16 of controller 18, such that ahigh signal will terminate the operation of switch 12. The over-voltageshut-down circuit 20 assures that transistors Q2 a and Q2 b are notdamaged in the event of an undesirably high voltage at the output of theac-to-dc converter; i.e., at point 40. As discussed above, there are anumber of factors which may cause high voltage conditions to exist.Lightning strikes or transients from other loads on the supply line,unregulated generators, runaway generators and the like may all causeover-voltage conditions.

Transistors Q2 a and Q2 b are rated at 500 volts. Because of theproperties of transformer T1, transistor elements Q2 a and Q2 b willexperience a voltage twice that imposed on capacitors C4 a, C4 b, and C4c. Accordingly, when capacitors C4 a, C4 b, and C4 c are at 250 volts,the transistor elements Q2 a and Q2 b will experience 500 volts.Accordingly, if the voltage of capacitors C4 a, C4 b, and C4 c exceeds250 volts transistors Q2 a and Q2 b may be damaged.

Operational amplifier U3B includes the following connections: pin 7 isthe output, pin 6 is the negative input, pin 5 is the positive input,pin 4 is connected to a 5 volt reference voltage 5REF, and pin 11 isconnected to ground. Pin 5 is connected to a voltage divider circuitcomprised of resistors R38, R39, and R40. The voltage applied to pin 5will vary depending on the line voltage of capacitors C4 a, C4 b, and C4c. Pin 6 is connected to a reference voltage through a voltage dividercircuit comprised of resistors R8, R7 and RN1B. Pin 7 is connected toresistor R40, and schottky diode D27 leading to shutdown pin 16 ofcontroller 18. The output of operational amplifier U3B will remain as alow, ideally zero, voltage and will not trigger shutdown via pin 16 ofcontroller 18 as long as pin 5 input does not exceed pin 6 input. Whenthe line voltage of capacitors C4 a, C4 b, and C4 c is sufficientlyhigh, the voltage on pin 5 will exceed the voltage on pin 6 and theoutput of pin 7 will trigger a shutdown.

The over-voltage shut-down circuit 20 will operate as follows under anormal voltage condition (i.e. a voltage condition that does not requirean over-voltage shutdown). Resistors RN1B and R7 form a voltage dividercircuit, where resistor RN1B is connected to a 5 volt reference 5REF andresistor R7 is connected to ground. Accordingly, pin 6 receives thevoltage between resistor RN1B and R7. Remember, that the voltage appliedbetween resistors RN1B and R7 will vary depending upon the operation ofthe over-temperature circuit 28 (i.e. when the temperature is coldresistor R8 is in parallel with resistor R7, and when a thermal shutdowntemperature is achieved resistor R8 is in parallel with resistor RB1B).Accordingly, the voltage applied to pin 6 will vary depending on whetheror not a thermal shutdown temperature is present. However, once athermal shutdown has been triggered by over-temperature shut-downcircuit 28 the operation of the over-voltage circuit 20 is irrelevant.Thus, for this explanation it will be assumed that the temperature isbelow shutdown level and resistor R8 is in parallel with resistor R7.

In the illustrated embodiment, the values of resistors R8, R7 and RN1Bare 499K, 32.4K, and 47.5K ohms respectively. Remembering that at a lowtemperature resistors R8 and R7 are in parallel, their equivalentresistance is 30.4K ohms. This resistance of 30.4K ohms will be calledR_(coldtemp). Accordingly, the voltage at pin 6 will be the measuredvoltage between resistor RN1B and R_(coldtemp). Using a voltage dividerthe voltage applied to pin 6 at a cold temperature is 1.925 volts. Thisvoltage will be called V_(shutdownref).

In illustrative embodiment, the value of resistors R38, R39, and R40 is84.5K, 866, and 97.6K ohms, respectively. Prior to an over-voltageshutdown, pin 7 will remain at a low, ideally zero, voltage, causingresistor R40 to be in parallel with resistor R39. The equivalentresistance of resistors R39 and R40 in parallel is 858.4 ohms. Thisresistance of 858.4 ohms will be called R_(normalvoltage). Pin 5receives the voltage between the voltage divider circuit created byresistors R38 and R_(normalvoltage). Accordingly, the unregulated DCterminal 40 voltage must exceed 195 volts for the voltage at pin 5 toexceed V_(shutdownref) (e.g., if unregulated DC terminal 40 carries avoltage of 195 volts, pin 5 will be at approximately 1.961 volts whichsufficiently exceeds the 1.952 volts applied to pin 6). Thus, when theunregulated DC terminal 40 reaches a voltage of 195 volts the output, ofpin 7 will become positive causing controller 18 to shutdown theswitching circuit 12. Because capacitor voltage is approximately 1.4times AC line voltage, the illustrative embodiment of the over-voltageshutdown circuit 20 will shut down the DC output if the AC input voltageexceeds 140 volts (i.e. the voltage of capacitors C4 a, C4 b, C4 cexceeds 195 volts). Remember that the preferred embodiment of thepermanent over-voltage indicator 22 will be triggered at about 220volts. Accordingly, the output of the switched power converter will beterminated by the over-voltage shut down circuit 20 at a lowerover-voltage condition than that which changes the state of the fuse Fx1in the permanent over-voltage indicator 22.

The over-voltage shutdown circuit 20 operates as follows under anover-voltage shutdown condition (i.e. the AC input voltage exceeds 140volts). When a over-voltage shutdown condition is reached, the voltageapplied to pin 7 is approximately 5 volts. When, pin 7 reaches 5 volts,resistor R40 is no longer in parallel with resistor R39, but will beused for a hysteresis effect. For example, when pin 7 is positive (i.e.over-voltage condition) resistor R40 will provide feedback into pin 5,which will in turn increase the voltage at pin 5. Accordingly, onceoperational amplifier U3B triggers a shut down, the voltage at terminal40 must be significantly lower than the 195 volts which triggered theinitial shut down because resistor R40 has temporarily increased thevoltage measured by pin 5. The purpose of the resistor R40 hysteresis isto prevent the controller 18 from operating the switching circuit 12until the voltage at terminal 40 has significantly fell below 195 volts.

Fan control Circuit

The fan control circuit 30 includes resistors RN1A, R4, RN1C, R2 a, R1,and R20, thermistor RT1, operational amplifier U3D, transistor Q1,capacitor C5, and Schottky Diode D1 a. FIGS. 2-16 are used to describethe operation of the fan control circuit 30 and the fan 32. In thisembodiment, the fan 32 is powered by a dc motor which varies in speed asa function of voltage amplitude; i.e., it is the control circuit whichproduces the variable speed characteristic. The fan control circuit 30commands the fan 32 to come on at an initial (lowest) temperature. Thespeed of the fan 32 increases with temperature and will maximize at somepoint prior to the switched power converter being at full load. The fancontrol circuit is also described in the aforementioned provisionalapplication, attorney docket no. PDY-106-A, the content of which isincorporated herein by reference.

An operating curve of the fan 32 using the fan control circuit 30 isshown in FIG. 3. (The slope is not necessarily linear as discussed inmore detail herein.) The fan control circuit 30 will cause the fan 32 tocome on at low speed when temperatures are over the set point by only asmall amount.

The relationship between the voltage applied to the fan and the fanspeed is shown in FIG. 4. Due to static friction the fan 32 does notstart moving until a certain voltage is reached. Specifically, and asillustrated in FIG. 4, the fan blades will not move until the voltage atpoint 2 is reached. Compared to the thermal time constants, it more orless instantaneously starts moving, jumping to point 3 (initial turn onpoint). As the voltage increases, it moves to point 4, where the fan 32is operating at maximum speed. On the way down, the variable voltagecontrolled fan 32 follows from point 4 (maximum operation) to point 3(initial turn on point) to point 1 (shut off).

FIG. 6 illustrates in principle how the fan control circuit 30 works.Temperatures T_(H) and T_(L) are the temperatures at which the fan 32 isideally full on and full off, respectively. More accurately, T_(H) (lineC) is the temperature at which full fan voltage is applied, and T_(L)(line D) is the temperature at which no voltage is applied to the fan32. Currents below point 14 have steady-state operating points on the“fan off” line (line B). currents above point 15 have steady-stateoperating points on the “fan full on” line (line A). Therefore, points14 and 15 must be the beginning and end of the line of operating pointswhen operating at currents where the variable voltage controlled fan 32is in an intermediate state between full on and full off. Although astraight line (line E) is shown connecting these two points, therelationship is not necessarily a linear one. It is clearly, however, astrictly increasing (positive slope) function. FIG. 6 illustrates theideal case.

Assume the switched power converter starts cold at current operatingpoint I_(OP2), point 1. The switched power converter will warm up and atpoint 2, T_(L), the fan 32 will start to turn slowly. The heat sink 52continues to warm up until it reaches its steady-state operating point,point 4. Similarly, for current operating point I_(OP1), the switchedpower converter will start at point 18, the fan 32 will come on at point6 and settle into a steady speed at point 7.

Turning to FIG. 4, the operating characteristics of fan 32 areexplained. Assume that T_(L1) corresponds to point 1 on FIG. 4 and thatT_(L2) corresponds to point 2 on FIG. 4 (same as point 3). Thus,returning to FIG. 6, line G describes an actual fan 32. Again, thisrelationship is not necessarily a linear one as shown, but it is apositive slope function. Starting cold with operating current I_(OP2),the temperature increases. At point 2 (T_(L)), voltage starts beingapplied to the variable voltage controlled fan 32, but it is not yetmoving. At point 3 (T_(L2)) the fan 32 begins to rotate. The switchedpower converter continues to heat up and eventually settles at point 5(along line G). For I_(OP1), the switched power converter would startcold at point 18 and heat up to point 6 (T_(L)). At point 6, voltagebegins to be applied to the fan 32. The switched power converter willcontinue to heat up until point 9 (T_(L2)), where the fan 32 beginsmoving. The fan 32 will now be moving faster than it needs to, theswitched power converter will cool and eventually settle into a steadystate speed at point 8 (along line G).

In both cases, the fan 32, once started, continues to rotate. There isno discontinuance of operation. Notice further that variable voltagecontrolled fan 32 speeds are slower (and less noisy) for all currentlevels up to point 15 (T_(H)). Also notice the minimum current to turnthe fan 32 on corresponds to point 17 (T_(L2)), but if already on, itwill stay on to a lower current, corresponding to point 16 (T_(L1)).

A description of the fan control circuit 30 is illustrated in FIGS. 2,and 7-16. The preferred embodiment of the fan control circuit includesresistors RN1A, R4, RN1C, R1, and R2 a, thermistor RT1, transistor Q1,and operational amplifier U3D. Operational amplifier U3D includes thefollowing connections: pin 14 is the output, pin 12 is the positiveinput, pin 13 is the negative input, pin 11 is connected to ground, andpin 4 is connected to a 5 volt reference voltage 5REF.

As illustrated in FIGS. 2, and 7, thermistor RT1 is used as atemperature sensor for the fan control circuit 30 as well as theover-temperature shutdown circuit 28. Thermistor RT1 is connected toground as well as resistor RN1A which also connected to a 5 voltreference 5REF. Thermistor RT1 and resistor RN1A are used to create avoltage divider circuit where V_(tempvar) is the output of the voltagedivider circuit. V_(tempvar) is connected to pin 13 of operationalamplifier U3D. Preferably RT1 is a negative-temperature-coefficientthermistor. As the internal temperature increases, V_(tempvar)decreases. For the remainder of the fan control circuit 30, a profile ofa desirable fan voltage versus V_(tempvar) is shown in FIG. 8.

Because the components used in switched power converter (i.e.operational amplifier U3D) are powered by 5 volts, whereas the fan 32requires a nominal 12 volts, a direct connection of an operationalamplifier such as that shown in FIG. 9 will not work. Simply stated anoperational amplifier such as operational amplifier U3D cannot supplysufficient current or voltage to the fan 32. Neither will transistoremitter follower-type circuits work because of voltage limitations. Anopen collector operational amplifier would work in a circuit such asthat shown in FIG. 10, and a simple gain amplifier would almost providethe desired profile as shown in FIG. 11. Shifting the “zero” point willget the desired profile as shown in FIG. 12. Specifically, a Theveninresistance and voltage coupled to the negative input of the operationalamplifier would shift the zero point of the fan control.

FIG. 13 illustrates an equivalent of the Thevenin resistance andvoltage, and the open collector operational amplifier is shownequivalently in FIG. 14. Using a conventional operational amplifierhaving an output connected to resistor R1 and transistor Q1 will resultin a complete fan control circuit according to FIG. 15. In almost allcases, the fan 32 will be quiet, and only under extended high load orhigh ambient temperature condition will the switched power converterwarm up enough to cause the fan 32 to be heard.

Because the circuit in FIG. 15 has a linear range between full on andfull off, significant power will be dissipated in transistor Q1 atintermediate fan speeds. An alternative is to modify the linear circuitto act as a duty cycle control circuit as shown in FIG. 16. With dutycycle control, transistor Q1 will be either full on or full off (zerovoltage or zero current), but the duty cycle will vary to control thespeed of the fan.

In FIG. 16, resistor R3 adds hysteresis and causes operational amplifierU3D to behave as a comparator. As the switched power converter warms up,transistor Q1 is off until it reaches a “low” temperature. The fancontrol circuit 30 then breaks into oscillation with low “on” duty cycleon transistor Q1. As the switched power converter continues to warm, theduty cycle gets larger. When an upper temperature is reached, theoscillation stops, and transistor Q1 is always on and stays on as thetemperature increases further.

The fan control circuit 32 as shown in FIG. 2 includes resistors RN1Cand R4 acting as a voltage divider circuit connected to pin 12 ofoperational amplifier U3D. Resistor RN1C is connected to 5 voltreference 5REF and resistor R4 is connected to ground. The preferredvalue of resistor RN1C is 9.53K ohms, and the preferred value ofresistor R4 is 22.6K ohms. More exactly, the currents flowing through R2a will also contribute to voltage at pin 12. Analysis yields

${{VP}\; 1\; N\mspace{14mu} 12} = \frac{\frac{{+ 5}\; {VREF}}{RNIC} + \frac{{VQ}\; 1\; C}{R\; 2\; a}}{\frac{1}{R\; 2\; a} + \frac{1}{RNIC} + \frac{1}{R_{4}}}$

where R2 a has the preferred value of 453K and VQ1C is the collectorvoltage of Q1. When Q1 is off and no current flows through the fan, VQ1Ccan be as high as the voltage in C5, which can vary with line voltage.

Using a nominal value of 15 volts for the voltage on C5 yields pin 12voltages;

VPIN12=3.4657 for VQ1 c=0 volts

VPIN12=3.6844 for VQ1 c=15 volts

Thus VPIN12 can more exactly have a range of voltages between 3.4657and'3.6844 depending on the voltage at the collector of Q1. At pin 13 ofoperational amplifier U3D, resistor RN2A and thermistor RT1 act as avoltage divider circuit. The preferred value of resistor RN1A is 16.2Kohms, and the preferred value of thermistor RT1 is 100K ohms at a coldstart up temperature (25° C.). Accordingly, the initial voltage appliedto pin 13 at a cold temperature is approximately 4.3 volts, which willbe called V_(tempvar).

At the initial startup of the switched power converter 46, V_(tempvar)is greater than 3.68v. Thus the output of operational amplifier U3D isnear zero causing transistor Q1 to be off and the fan 32 is not running.As the temperature increases, the resistance of thermistor RT1 willdecrease causing the value of V_(tempvar) to drop from the initial 4.3volts. Eventually the temperature will increase such that the value ofV_(tempvar) will fall slightly below 3.68 volts. When this occurs, thecircuit including operational amplifier U3D will enter the linearregion. There will be a slight fan voltage but it will probably remainin the stalled condition. If the temperature continues to increase thevalue of V_(tempvar) will fall significantly below 3.68v but above 3.46vand operational amplifier U3D causes the fan to enter the mid speedrange. As V_(tempvar) falls further, op-amp U3D turns transistor Q1 fullon and the fan 32 reaches full speed.

The fan control circuit 30 provides the variable voltage to control thespeed of the fan 32. The transformer circuit 14 provides steady power tothe power input of the fan 32. The power input for the fan 32 isconnected to pin 3 of transformer T1, through Schottky diode D1 a andresistor R20. Pin 7 of transformer T1 is connected to ground, completingthe power input circuit for the fan 32. Resistor R20 is used for thepurpose of preventing the voltage applied to the fan 32 from exceedingspecifications. One plate of capacitor C5 is connected to ground and theother plate is connected between resistor R20 and Schottky diode D1 afor the purpose of providing a steady voltage to resistor R20. CapacitorC5 is charged by transformer T1 and carries enough voltage to power thefan 32. Schottky diode D1 a prevents capacitor C5 from discharging intopin 3 of transformer T1.

Because the power input to the fan 32 is connected to the primary sideof the transformer circuit 14, the fan control circuit 30 as well as thevariable voltage controlled fan 32 will remain operational even when theoutput is heavily loaded or short circuited. Simply stated, this featurewill permit the cooling system of the switched power converter tocontinue to operate in the event of an over-loaded output.Appropriately, the occurrence of this condition is when the operation ofthe fan 30 is most vital.

Transformer Circuit

Transformer circuit 14 is primarily inclusive of transformer T1. Pin 4of transformer T1 is the positive input fine. Pin 4 is connected tounregulated DC terminal 40, where the DC line voltage is approximately170 volts. Pins 5 and 6 of transformer T1 are connected to the switchingcircuit 12. The switching circuit 12 provides a switching current to theprimary-input side of transformer T1. For example, switching circuit 12,which is controlled by controller 18, allows current to flow betweenpins 4 and 5, and between pins 4 and 6. However, the current betweenpins 4 and 5, and pins 4 and 6 will never flow simultaneously, but willalternate according to controller 18. Operation is described below withreference to FIG. 17.

Switching Circuit

As illustrated by FIGS. 2 and 17, the preferred embodiment of theswitching circuit 12 contains two transistors Q2 a and Q2 b. Whentransistor Q2 a is turned on current I₁ will flow from pin 4 oftransformer T2 to pin 6. Alternatively when transistor Q2 b is turned oncurrent I₂ will flow from pin 4 of transformer T1 to pin 5. Whentransistor Q2 a is on transistor Q2 b will be off, and when transistorQ2 b is on, Q2 a will be turned off. The primary-input side oftransformer T1 is utilized in such a fashion so that the transistorswithin the switching circuit 12 may operate at up to a maximum 50% dutycycle, meaning that, transistors Q2 a and Q2 b are never on more than50% of the time.

As further illustrated in FIGS. 2 and 17, the secondary-output side oftransformer T1 includes pins 2, 8, and 1. When current I₂ flows betweenpins 4 and 5 of transformer T1, current I₄ will correspondingly flowbetween pins 8 and 2. Alternatively, when current I₁ flows between pins4 and 6 of the primary-input side of transformer T1, current I₃ willcorrespondingly flow between pins 8 and 1 of the secondary-output side.The switching circuit 12 further includes R37, R23 a, R23 b and C24(shown only in FIG. 2).

Transistors Q2 a and Q2 b provide two current loops. Transistor Q2 a isconnected to pin 6 on the primary side of transformer T1, and transistorQ2 b is connected to pin 5 on the primary side of transformer T1.Controller 18 controls the on/off state of transistors Q2 a and Q2 b.When transistor Q2 a is turned on, Q2 b is off. Current I₁ flows betweenpins 4 and 6 of transformer T1; alternatively, when transistor Q2 b isturned on, Q2 a is off and current I₂ flows between pins 4 and 5 oftransformer T1. The gate of transistor Q2 a is connected to resistor R23a which is connected to AOUT (pin 11) on controller 18. The gate oftransistor Q2 b is connected to resistor R23 b which is connected toBOUT (pin 14) on controller 18. When controller 18 applies a voltage tothe gate of transistor Q2 a, transistor Q2 a turns on and allow currentI₁ to flow from pin 4 of transformer T1, through pin 6, and then toground through the drain and source of transistor Q2 a. Alternatively,when controller 18 applies a voltage to the gate of transistor Q2 b,transistor Q2 b will turn on and allow current I₂ to flow from pin 4 oftransformer T1, through pin 5 and to ground through the drain and sourceof transistor Q2 b .

Resistor R37 and capacitor C24 are connected in series between the drainof transistors Q2 a and Q2 b for the purpose of snubbing the transientdrain voltage when transistors Q2 a and Q2 b are switching.

Controller

Controller 18 is used for controlling the output of the switchingcircuit, 12 by controlling the duty cycles of switching transistors Q2 aand Q2 b. Controller 18 receives input from the current sensing circuit34, over-voltage shut-down circuit 20, over-temperature shut-downcircuit 28, feedback circuit 16, and foldback circuit 42.

As discussed previously, AOUT (pin 11) and BOUT (pin 14) are connectedto transistors Q2 a and Q2 b respectively for the purpose of controllingthe duty cycle and switching current of the switching circuit 12. SHDN(pin 16) is connected to both the output of the over-voltage shutdowncircuit 20 and the over-temperature shutdown circuit 28 for the purposeof terminating the operation of the switching circuit 12. If SHDN (pin16) receives a sufficient voltage AOUT (pin 11) and BOUT (pin 14) willturn off transistors Q2 a and Q2 b, which will terminate the outputacross the DC load.

CS+ and CS− are connections to operational amplifier CS, which isinternal to controller 18. The output of operational amplifier CScorresponds to the instantaneous voltage output of current sensingcircuit 34. CS+ (pin 4) is connected to the output of the currentsensing circuit 34, which measures the. current through transistors Q2 aand Q2 b.

EA+, EA−, and COMP are connections on operational amplifier EA, which isinternal to controller 18. The output of operational amplifier EA iscompared to the output of operational amplifier CS. EA+ (pin 5), isconnected to the output of the feedback circuit 16. EA− (pin 6) isconnected to COMP (pin 7), acting as a voltage follower on operationalamplifier EA. Accordingly, the output of operational amplifier EA willbe the same as the voltage applied to EA+ (pin 5).

If the instantaneous output of operational amplifier CS exceeds theoutput of operational amplifier EA AOUT (pin 11) and BOUT (pin 14)transistors Q2 a and Q2 b are turned off. If the current generated bytransistors Q2 a and Q2 b exceeds the limit set by feedback circuit 16,controller 18 will temporarily terminate the gate drives to Q2 and Q2 b.This comparison/control function occurs on a cycle-by-cycle basis.

CLADJ (pin 1) is used to further limit the current output of theswitching circuit 12. The voltage applied to CLADJ (pin 1) limits themaximum current output of the switched power converter. As the voltageapplied to CLADJ (pin 1) decreases so does the maximum current output ofthe switched power converter. CLADJ (pin 1) is connected to the outputof the foldback circuit 42, where the foldback circuit will cause thecurrent limit to decrease (i.e. reduce the voltage applied to CLADJ) ina near short circuit situation. CLADJ (pin 1) is also connected betweenresistors R14 and R15 which act as a voltage divider circuit. ResistorR14 is connected to 5 volt reference 5REF and is in series with resistorR15. Resistor R15 is also connected to ground.

VREF (pin 2) provides a 5.1 volt reference voltage which supplies powerto various electrical components within the switched power converter.The output of VREF is identified as 5 volt reference 5REF. VIN (pin 15)is connected to a power supply for the purpose of providing power tocontroller 18. VIN (pin 15) is connected to Zener diode D9 and capacitorC10 which provide approximately 15 volts to controller 18. Zener diodeD9 and capacitor C10 receive voltage from unregulated DC terminal 40through resistors R24 a and R24 b.

VC (pin 13) is the power supply for the sales of transistors Q2 a and Q2b through AOUT (pin 11) and BOUT (pin 14), respectively. VC (pin 13) isconnected to VIN (pin 15) through resistor R16. Resistor R16 is used tolimit the current entering VC (pin 13). Schottky diodes D16 a, D15 a,D15 b and D16 b are used to prevent the voltage on AOUT (pin 11) andBOUT (pin 14) from exceeding VIN or from dropping below GND.

GND (pin 12) is connected to ground. Capacitor C25 is connected to CT(pin 8) and resistor R13 is connected to RT (pin 9) for setting thefrequency and maximum duty cycle of controller 18. Capacitor C25 andresistor R13 are also connected to ground. SYNC (pin 10) is notutilized.

Foldback Circuit

As briefly mentioned, foldback circuit 42 provides feedback tocontroller 18 for the purpose of reducing the duty cycle of transistorsQ2 a and Q2 b under near short circuit conditions rather than allowingthe output current across the DC toad to increase out of control.Foldback circuit 42 includes, diode D4, resistors R19 a, R19 b, R17, andR18, capacitor C8, and operational amplifier U3C. Operational amplifierU3C has the following connections: pin 8 is the output, pin 9 is thenegative input, pin 10 is the positive input, pin 4 is connected to 5volt reference 5REF, and pin 11 is connected to ground.

The foldback circuit 42 measures the duty cycle of transistors Q2 a andQ2 b. Pin 10 is connected to AOUT (pin 11) and BOUT (pin 14) oncontroller 18 through resistors R19 aand R19 b.

Capacitor C8, which is connected between pin 10 and ground, as well asin series with resistors R19 a and R19 b is used for the purpose ofaveraging the duty cycle controlled gate voltages of transistors Q2 aand Q2 b. Resistor R17 is connected between 5 volt reference 5REF andpin 10, and resistor R18 is connected between pin 10 and ground for thepurpose of creating a voltage divider circuit to reduce the voltageapplied to pin 10. Pin 9 is connected to pin 8 for the purpose ofcreating a voltage follower, such that the voltage at pin 8 will alwaysequal the voltage applied to pin 10. Pin 8 is also connected to thecathode of diode D4, and the anode of diode D4 is connected to CLADJ(pin 1) of controller 18.

As the duty cycle of AOUT (pin 11) and BOUT (pin 14) increases, thevoltage of capacitor C8 increases as well as the voltage on pin 10.Accordingly, the voltage on pin 8 will be higher than the voltagebetween resistors R15 and R14. When this occurs, diode D4 will bereverse biased and the voltage at CLADJ (pin 1) of controller 18 willnot be affected. In this situation the current limit of CLADJ willneither decrease nor increase because foldback circuit 42 is not pullingcurrent from CLADJ (pin 1).

As the duty cycle of AOUT (pin 11) and BOUT (pin 14) decreases, thevoltage of capacitor C8 decreases as well as the voltage on pin 10.Accordingly, the voltage on pin 8 will be lower than the voltage betweenresistors R15 and R14. When this occurs, diode D4 will be forward biasedand the voltage at CLADJ (pin 1) of controller 18 will be pulled down.As the voltage applied to CLADJ (pin 1) decreases, the maximum currentoutput of controller 18 will also decrease. Accordingly, in the event ofa near short circuit at the DC load, the reduced current limitation ofCLADJ will prohibit the current output from going unreasonably high andreduce the output current to less than its previous maximum rating.

Voltage Feedback Circuit

The feedback circuit 16 measures the voltage across the DC load andoutputs a reference voltage to controller 18. Controller 18 contains aninternal voltage controller, for the purpose of providing a voltagecontrolled current source. Controller 18 will control the switching oftransistors Q2 a and Q2 b accordingly. Feedback circuit 16 includesresistors R28, R34, R32, R26, R25, R33 and R30, capacitors C27, C22,C20, and C28, and optical coupler U2 which includes a LED, aphoto-sensor and a 2.5 volt reference.

When the DC load is increased, there is an immediate drop in voltageacross the DC output terminals of the power converter. This drop involtage requires an increase of output current in the output circuit 44in order to meet the new load demands. Alternatively, when the DC loadis decreased, there is an immediate increase in voltage. This increasein voltage requires a decrease in the output current of the outputcircuit 44 in order to compensate for the load reduction.

For example, when the operator of the switched power converter brings anadditional load on-line, the feedback circuit 16 first measures thevoltage across the load and then scales the voltage down to a 2.5 voltrange. Because a new load has been added the measured voltage will bebelow the 2.5 voltage range. Optical coupler U2 will compare themeasured voltage (scaled down) against a 2.5 volt reference. Because themeasured voltage across the load will be below the 2.5 referencevoltage, optical coupler U2 will cause the LED to produce less light.When the LED produces less light the photo-sensor will cause the outputof the feedback circuit to increase in voltage. The output of thephoto-sensor is connected to EA+ (pin 5) on controller 18. When thevoltage input of EA+ (pin 5) increases, the voltage controller withincontroller 18 will temporarily increase the duty cycle of the switchingcircuit 12. This in turn increases the load current to meet the new loaddemand (i.e. get the voltage across the DC load back up to 13.6 volts).

Alternatively, when the operator of the switched power converter removesa load, the feedback circuit 16 measures the voltage across the load andthen scales the voltage down to a 2.5 volt range. Because a load hasbeen removed the measured voltage will be above the 2.5 voltage range.Opto-coupler U2 will compare the measured voltage (scaled down) againsta 2.5 volt reference. Now, because the measured voltage across the loadwill be above the 2.5 reference voltage, opto-coupler U2 will cause theLED to produce additional light. When the LED produces additional lightthe output of the feedback circuit will decrease in voltage. The outputof the photo-sensor is connected to EA+ (pin 5) on controller 18. Whenthe voltage input of EA+ (pin 5) decreases, the voltage controllerwithin controller 18 will temporarily decrease the duty cycle of theswitching circuit 12. This in turn, decreases the load current to meetthe reduced load demand (i.e. get the voltage across, the DC load backdown to 13.6 volts).

Resistor R25 limits current to opto-coupler U2. Resistor R26 and R28 arearranged as a voltage divider to provide a scaled output voltage in thevicinity of 2.5 volts. Capacitors C20, C22, C27, C28, R34, R32 and R33are used for stability, do not affect the DC levels whatsoever as theycarry no DC current. Resistor R30 is used for providing an input voltageto EA+ (pin 5) of controller 18 based on the current output ofopto-coupler U2.

Current Sensing Circuit

Current sensing circuit 34 is used to measure the current being drawn bytransistors Q2 a and Q2 b and to send the measured current to CS+ (pin4) of controller 18. Controller 18 then compares this measured currentto a reference level. The reference level is the output of feedbackcircuit 16, which is connected to EA+ (pin 5) on controller 18.Depending upon the measured current and the reference level, controller18 will control the on/doff state of transistors Q2 a and Q2 b.

Current sensing circuit 34 includes transformer T4, diodes D24 a, D24 b,D24 c, and D24 d, resistors R21, R21 a, R21 b, R21 c, R21 d, and R21 e,and capacitor C9. The drain of transistor Q2 b is connected to pin 4 oftransformer T4, and the drain of transistor Q2 a is connected pin 6 oftransformer T4. The output side of transformer T4 (pins 1 and 2) isconnected to a series of diodes and resistors and then to CS+ (pin 4) ofcontroller 18.

Diodes D24 a, D24 b, D24 c, and D24 d make up a full wave rectifierbridge. Diodes D24 c and D24 b are connected in parallel to the outputside of transformer T4, where the cathode of diode D24 c is connected topin 1 of transformer T4 and the cathode of diode D24 b is connected topin 2 of transformer T4. The anodes of diodes D24 b and D24 c are bothconnected to ground. Diodes D24 d and D24 a are also connected inparallel to the output side of transformer T4, where the anode of diodeD24 d is connected to pin 1 of transformer T4 and the anode of diode D24a is connected to pin 2 of transformer T4. The cathodes of diodes D24 aand D24 d are connected to CS+ (pin 4) of controller 18 as well as aseries of resistors and a capacitor.

For example, when transistor Q2 a is turned on, the current fromtransformer T4 will flow from pin 1 of the transformer, through diodeD24 d and through resistors R21, returning through D24 b to pin 2. Avoltage representing the flow of this current trough R21 is connected topin 4 of CS+ in controller 18. When transistor Q2 b is turned on, thecurrent from transformer T4 will flow from pin 2 of the transformerthrough D24 a and through R21 (to ground) and then returning through R24c to ground to pin 1 of T4. Again, the voltage on R21 resistors is fedto CS+, the op-amp in controller 18.

Resistors R21, R21 a, R21 b, R21 c, R21 d, and R21 e, and capacitor C9are all connected in parallel. The current output of diodes D24 d andD24 a are connected to the high side of resistors R21, R21 a, R21 b, R21c, R21 d, and R21 e, and capacitor C9. The low side of resistors R21,R21 a, R21 b, R21 c, R21 d, and R21 e, and capacitor C9 are connected toground. This parallel resistor-capacitor circuit is used for the purposeof ensuring the voltage applied to CS+ (pin 4) of controller 18 is inthe 1 volt range.

Output Circuit

The secondary-output side of transformer T1 is connected to the DC loadthrough a series of circuit elements making up the output rectifier andLC filter circuit 44. The output circuit 44 includes capacitors C19,C11, C13A, C12, C14A, C17, and C18, schottky diodes D11 a and D11 b,diode D12, resistor R29, inductor L2, fuses F2, F3, and F4, inductorbeads L3, L4, L10, and L11, and heavy gauge wires 105, 106, and 107. TheDC load is connected in parallel with capacitors C11, C13A, C12, C14A,C17, and C18 (DC load capacitors), which are in series with inductor L2.The output circuit 44 is integral with the transformer secondary andincludes two current loops with the current going in the same directionthrough inductor L2, the DC load capacitors, and the DC load (FIG. 17).

As further illustrated by FIG. 17 when transistor Q2 a is turned on (Q2b is off) current I₁ will flow between pin 4 and pin 6 (primary-inputside of transformer T1) in a counter-clockwise direction. Current I₁will cause current I₃ to flow between pin 8 and pin 1 (secondary side oftransformer T1) in a clockwise direction. Alternatively, when transistorQ2 b is turned on (Q2 a is off) current I₂ will flow between pin 4 andpin 5 (primary-input side of transformer T1) in a clockwise direction.Current I₂ will cause current I₄ to flow between pin 8 and pin 2(secondary side of transformer T1) in a counter-clockwise direction. Asillustrated the current (I₄ and I₃) applied to inductor L2 is alwaysgoing in the same direction.

Further explained, when transistor Q2 a is turned on, current flows inthe secondary-output side of transformer T1 from pin 1 through schottkydiode D11 a, through inductor L2. The DC load capacitors will be chargedand current will be delivered to the DC load and back through pin 8 ofthe transformer. When transistor Q2 b is turned on, current flows in thesecondary side of transformer T1 from pin 2 through schottky diode D11b, through inductor L2, the DC load capacitors will be charged andcurrent will be delivered to the DC load and then back through pin 8 ofthe transformer.

Resistor 29 and capacitor C19 are connected in series betweensecondary-output pins 2 and 1 of transformer T1 for the purpose ofeliminating transient voltages. Inductor beads L3, L4, L10, L11, areconnected between the secondary-output side of transformer T1 andschottky diodes D11 a and D11 b. Inductor beads L3, L4, L10 and L11 areplaced on the leads of D11A and D11B, for the purpose of reducingtransient noise. The DC load capacitors which are connected in parallelwith the DC load are arranged as follows. Capacitor C11 is the mainoutput capacitor. The positive plate of capacitor C11 is connected tothe positive terminal of the DC load P4 and the negative plate isconnected to the negative terminal of the DC load P1.

The remaining capacitors are used for the purpose of reducing noise.Capacitor C12 is connected in parallel with the DC load, where one plateof capacitor C12 is connected to the positive terminal of the. DC loadP4, and the other plate of capacitor C12 is connected to the negativeterminal of the DC load P1. Capacitors C13A and C14A are connected inseries, where one plate of capacitor C13A is connected to the positiveterminal of the DC load P4, and one plate of capacitor C14A is connectedto the negative terminal of the DC load P1. The remaining plates ofcapacitors C13A and C14A are connected to chassis ground. Capacitors C17and C18 are also connected in series, where one plate of capacitor C17is connected to the positive terminal of the DC load P4, and one plateof capacitor C18 is connected to the negative terminal of the DC loadP1. The remaining plates of capacitors C17 and C18 are connected tochassis ground. Fuses F2, F3, and F4 are connected in series withinductor L2 and work in conjunction with diodes D11 a and D11 b toprovide reverse battery protection.

The illustrated embodiment of this invention also includes the use ofheavy gauge wires which supplement the copper laminations on the circuitboard. Heavy gauge wires 105 are connected directly between the negativeoutput (terminal 8) of transformer T1 and the negative terminal of DCload P1 (i.e. DC negative output 88) Heavy gauge wires 106 are connecteddirectly between schottky diodes D11A and the input of inductor L2.Heavy gauge wires 106 are also connected directly between schottkydiodes D11B and the input of inductor L2. Heavy gauge wires 107 areconnected directly between the output of inductor L2 and fuses F2, F3,and F4. The output of fuses F2, F3, and F4 are connected to the positiveterminal of DC load P4 (i.e. DC positive output 90).

Waveforms

FIG. 5 illustrates waveforms found at various points in the circuit ofFIG. 17 under normal operating conditions. FIG. 5A shows the voltagesacross the two power transistors Q2 a and Q2 b during a complete cycleof operation. One voltage is the complement of the other. FIG. 5B showsthe voltages across the primary windings of transformer T1 during onecomplete cycle of switch operation. FIG. 5C illustrates the currentwaveforms I₁ and I₂ through the primary loops of FIG. 17.

FIG. 5D shows the current through inductor L2.

FIG. 5E shows the secondary current through diode D11 a.

FIG. 5F shows the secondary current 14 through diode D11 b.

FIG. 5G shows the current through C11.

FIG. 5H shows the voltage at the top of the circuit of FIG. 17; i.e.,the top of L2.

Permanent Reverse Battery Indicator

The permanent reverse battery connection indicator 24 is diode D12.Diode D12 and capacitor C11 are connected in parallel. The cathode ofdiode D12 is connected to the positive plate of capacitor C11 which isconnected to the positive terminal of the DC load P4. The anode of diodeD12 is connected to the negative plate of capacitor C11 which isconnected in to the negative terminal of the DC load P1. If a reversebattery connection is applied to the DC load output of the powerconverter, diode D12 will blow before fuses F2-F4 open circuit,permanently indicating that a reverse battery connection has occurred.If F2-F4 blow, they may be replaced or reset and the converter 46 willbe fully operational even if D12 is not replaced.

Packaging a Commercial Device

Having described the preferred power conversion circuit, the packagingof a commercial embodiment will be described in detail with reference toFIGS. 18-26.

The commercial embodiment of converter 46 comprises a rectangular sheetmetal housing 70 attached by screws to a finned aluminum extrusion 52which forms the aforementioned heat sink for the FET's Q2 a and Q2 b,diode D11 a and D11 b,and the thermistor RT1. These components are heldagainst a large flat surface 53 of heat sink 52 by spring clips 55 whichare screwed into the heat sink extrusion in the manner shown in FIG. 19.The fan 32 is mounted by screws 57 onto an end of the heat sinkextrusion 52 in which a relief 59 of circular design has been machined.The surfaces of the relief 59 lie below the end surfaces 61 of the fins65 and the screw base 63 on which the fan 32 is mounted. This reliefcreates an air gap between the fan motor 50 and the heat sink whichprevents heat from the sink reaching the fan motor. Numerous vents 58are formed in the top and back plates of the housing 70.

Flanges 84 are provided on both ends of housing 70 for mountingpurposes. Fuses F2-F4 are mounted outside the housing 70 for ease ofreplacement. Fuse Fx1, however, is inside the housing for reasonsdescribed above. The positive output terminals 90 and the negativeoutput terminals 88 are mounted on the left side of housing 70 as shownin FIG. 22. A power cord 98 extends from housing 70 through aperture100.

The components in the circuit of FIG. 2 are mounted on a conventionalcircuit board 102 which is secured by fasteners within housing 70. Theboard 102 has conductive traces on both sides as shown in FIGS. 25 and26. The inductor L2 is mounted on board 102 as shown in FIG. 25 alongwith the transformer T1.(central in FIG. 25). Two No. 12 gauge wires 104run from the center tap of T1 to a point 106 where they pass through ahole in board 102 and emerge on the other side as shown in FIG. 26. Fromthere to the negative output terminal 88 the wires overlie a coppertrace and are soldered to the trace to lower the resistance of this highcurrent path and increase the robustness of it as well. The leads 108from L2 to the fuses F2-F4 and the positive outputs 90 are similarlyconstructed.

FIG. 23 shows the converter 46 mounted within an RV 109 having a storagebattery 114. A power cord 112 brings 115 vac to the converter from apedestal 111 of the type found in RV parks. The converter 46 isconnected into the electrical system of the RV in a known manner.

Referring again to FIG. 2 the circuit for the converter 46 is hereequipped with a 4-wire terminal H2 of which pin 4 is connected to theconverter output fuses. F2-F4 via a 100 Ohm resistor R57. The terminalH2 allows the converter to be connected to an external “management”system of the type described in U.S. Pat. No. 5,982,643 issued Nov. 9,1999 to Thomas H. Phlipot and assigned to Progressive Dynamics, Inc. Asis more completely described in the '643 patent, the management systemincludes a microcontroller which gives the owner the option of variousoperating modes and various converter output voltages; e.g., 13.6 v fornormal operation, 13.2 v for storage, and 14.4 v for boost.

Miscellaneous—Options

FIG. 2 also illustrates a terminal H4 connected to ground via R51, R31and C21. Terminal H4 is a two-contact terminal which is shorted out witha small bridge wire if a gel cell is used in place of the normallead-acid liquid storage battery 114 in the RV. This lowers theoperating voltages of the converter 46 by 0.4 v and is a convenientoption for owners who wish to use gel cell storage batteries

While the present invention has been described in connection with whatis presently considered to be the most practical and preferredembodiment, it is to be understood that the invention is not to belimited to the disclosed embodiments but, on the contrary is intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, which scope is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures as is permitted under the law.It is also to be understood that it is the inventor's intent to claimall novel subject matter contained within this disclosure.

VALUES OF LISTED COMPONENTS C1 Capacitor 0.47 uF C10 Capacitor 220 uFC11 Capacitor 16 V C12 Capacitor 0.01 uF C13A Capacitor 0.01 uF C14ACapacitor 0.01 uF C15 Capacitor 2.2 nF C16 Capacitor 2.2 nF C17capacitor 0.047 uF C18 capacitor 0.047 uF C19 capacitor 0.001 uF C2capacitor 2.2 nF C20 capacitor 0.015 uF C22 capacitor 0.47 uF C24capacitor 270 pF C25 capacitor 0.12 uF, 2% C26 capacitor 0.47 uF C27capacitor 1000 pF C28 capacitor 0.1 uF C29 capacitor 2.2 nF C3 capacitor2.2 nF C30 capacitor 0.47 uF C31 capacitor 2.2 nF C4a capacitor 820 uF,250 V C4b capacitor 820 uF, 250 V C4c capacitor 820 uF, 250 V C5capacitor 100 uF, 35 V  C7 capacitor 0.1 uF C8 capacitor 0.1 uF C9capacitor 0.01 uF D10 zener diode 13 V D11a schottky diode 40 A, 100 VD11b schottky diode 40 A, 100 V D12 diode D15a schottky diode 1 A, 20 VD15b schottky diode 1 A, 20 V D16a schottky diode 1 A, 20 V D16bschottky diode 1 A, 20 V D1a schottky diode  1 A, 100 V D23 zener diode220 V, 5 W, 5% D24a diode 75 V, 150 mA D24b diode 75 V, 150 mA D24cdiode 75 V, 150 mA D24d diode 75 V, 150 mA D27 schottky diode 1 A, 20 VD3 schottky diode 1 A, 20 V D4 diode 75 V, 150 mA D9 zener diode 15 V, 2W DB1 diode bridge 20 A, 400 V Bridge F1 fuse 15 A F2 fuse 30 A F3 fuse30 A F4 fuse 30 A Fx1 fuse 0.5 A L2 inductor 20 uH Q1 transistor 5 A, 40V Q2a transistor 24 A, 500 V, .20 on resistance Q2b transistor 24 A, 500V, .20 on resistance R1 resistor 390 Ohm, 5%  R13 resistor 1.82 K R14resistor 16.2 K R15 resistor 35.7 K R16 resistor 1.8 K, 5% R17 resistor5.49 K R18 resistor 15.4 K R19a resistor 12.1 K R19b resistor 12.1 K R20resistor 50 Ohm, 5%, 3 W R21 resistor 18.7 Ohm R21A resistor 422 OhmR21B resistor 422 Ohm R21C resistor 845 Ohm R21D resistor 845 Ohm R21Eresistor 1690 Ohm R23a resistor 15 Ohm, 5% R23b resistor 15 Ohm, 5% R24aresistor 1.5 K, 5%, 10 W R24b resistor 1.5 K, 5%, 10 W R25 resistor 1 K,5%, ½ W R26 resistor 31.2 K or 30.1 K R28 resistor 6.98 K, ¼% R29resistor 10 Ohm, 5% R2a resistor 453 K R30 resistor 4.7 K R33 resistor3.24 K R34 resistor 3.24 K R37 resistor 100 Ohm, 5%, 10 W R38 resistor84.5 K, 0.5 W R39 resistor 866 Ohm R4 resistor 22.6 K R40 resistor 97.6K R7 resistor 32.4 K R8 resistor 499 K RN1A resistor 16.2 K RN1Bresistor 47.5 K RN1C resistor 9.53 K RT1 thermistor 100 K RT2 thermistor1 Ohm T1 transformer 2:13:13:2:2 T2 CMC transformer custom T3 CMCtransformer custom T4 transformer 80 MH U2 optically isolated amplifierFOD2741 USA operational amplifier LM2902 U3B operational amplifierLM2902 U3C operational amplifier LM2902 U3D operational amplifier LM2902

1. A power converter having an input for receiving a supply voltage, aswitch, and a duty cycle controller connected to control the switch toregulate an output voltage produced by the converter wherein theimprovement comprises: a first circuit connected to said input andresponsive to a first level over-voltage condition at said input foraltering operation of duty cycle controller; and a second circuitconnected to said input and responsive to a second level over-voltagecondition at said input to irreversibly change state without alteringoperation of the duty cycle controller.
 2. A converter as described inclaim 1 wherein the second level is higher than the first level.
 3. Apower converter as described in claim 1 wherein the first circuitcomprises an amplifier, circuit connected to said input and to areference voltage, said amplifier circuit having an output connected tosaid controller to shut, said controller down in response to the firstlevel over-voltage condition.
 4. A power converter as described in claim1 wherein the second circuit comprises the series combination of anon-resettable fuse and a Zener diode connected between said input andground.
 5. A power converter having an input connectible to an AC linevoltage source, a rectifier for converting an AC line voltage to DC, aswitching circuit comprising at least one power transistor, atransformer having a primary side connected to said rectifier and tosaid switch circuit and a secondary side connected to a converteroutput, a controller circuit for operating said switch circuit in avariable duty cycle mode wherein the improvement comprises: a permanentover-voltage indicator circuit connected between said rectifier andground and including the series combination of a voltage responsivesemi-conductor device and an element which irreversibly changes state inresponse to the presence of an over-voltage condition at said rectifieroutput.
 6. The power converter described in claim 5 wherein said elementis a fuse.
 7. The power converter described in claim 5 wherein saidsemi-conductor device is a Zener diode.
 8. The power converter of claim5 further comprising an over-voltage shut-down circuit connected betweensaid rectifier output and said controller for shutting the controllerdown under over-voltage conditions.
 9. The power converter of claim 8wherein said over-voltage shut-down circuit further includes anoperational amplifier.
 10. A power converter comprising an input forreceiving a supply voltage, a switch including at least one powertransistor, and a duty cycle controller connected to cycle the switch onand off; an over-voltage shut-down circuit connected between said inputand said controller and comprising an operational amplifier; saidoperational amplifier having an output, connected to a shut down pin ofsaid controller to terminate cycling of the switch.
 11. A powerconverter for charging a battery and having a switch, a duty cyclecontroller connected to the switch, a transformer having a primary sideand a secondary side, an output inductor, the primary side beingconnected to the switch and the secondary side being connected to theoutput inductor, a circuit for indicating a reverse battery connectionand comprising a device connected between the output inductor and groundand effective to irreversibly change state when a reverse batteryconnection is made.
 12. A power converter as defined in claim 11 furtherincluding a capacitor connected in parallel with said device.
 13. Apower converter as defined in claim 12 wherein the device is a Schottkydiode.
 14. A power converter comprising a switch, a controller forcontrolling the duty cycle of the switch, a transformer, an outputinductor and a converter output terminal; a circuit board havingconductive traces formed thereon, the inductor being mounted on thecurrent board such that at least one conductive trace runs from theinductor to the output terminal; and a heavy gauge wire conductivelybonded to and along said one conductive trace.
 15. A power convertercomprising a switch, a controller for controlling the duty cycle of theswitch, a transformer, an output inductor and a converter outputterminal; a circuit board having conductive traces formed thereon, atleast one trace running from the transformer to the output terminal; anda heavy gauge wire conductively bonded to and along said one conductivetrace.
 16. A power converter including a switch, a controller forvarying the duty cycle of the switch, a heat sink for the switch, a fanfor causing the fan to cause air to flow over the heat sink; a heatsensor; and a fan control circuit, the fan control circuit beingoperative to control the supply voltage source to vary the speed of thefan in response to increasing temperature of said sensor.
 17. The powerconverter defined in claim 16 wherein the fan comprises a motor mountedproximate the heat sink, the heat sink comprises a metal extrusionhaving a recess machined into an end thereof to provide an air gapbetween the heat sink and the fan motor.